The present invention concerns an electric current sensor for measuring a current flowing through a primary conductor, this sensor comprising a magnetic circuit arranged to be coupled with said primary conductor, a secondary winding coupled with this magnetic circuit, a damping resistance of said winding, a magnetic field detector arranged within an air gap of said magnetic circuit, a current supply device for said secondary winding, where this current supply device is controlled by the magnetic field detector so as to feed to the secondary winding a compensating current which tends to compensate the magnetic field created in the air gap by the primary current, and a measuring resistor connected in series with the secondary winding where the voltage drop across this resistor forms the measuring signal of the sensor.
In sensors of this type the damping resistance associated with the secondary winding will influence the result of the measurement depending on the frequency range in which the current to be measured and its harmonic components are situated. In fact, in the low-frequency range, that is, from direct current up to a transition frequency, the sensor works in a compensating mode where a compensating current is injected into the secondary winding so as to compensate by the magnetic field created by this compensating current, the magnetic field induced by the primary current in the air gap of the magnetic circuit. The current flowing in the measuring resistor thus has a component due to the presence of the damping resistance of the secondary winding which adds to the compensating current in the strict sense, that is, to the component which, if there was no internal resistance of the winding, would suffice to compensate the magnetic field induced by the primary current. Thus, the measurement exhibits a positive error in the low-frequency range.
To the contrary, in the range of frequencies higher than the aforementioned transition frequency and up to a high cut-off frequency, the sensor will work as a current transformer. In this case the current flowing in the measuring resistor is lower than the exact value of the primary current, because of the damping resistance present in the secondary winding, so that the measurement is affected by a negative error.
The present invention has the particular aim of correcting the measuring error caused by the ohmic resistance of the secondary winding and by the damping resistance of this winding in both of the cases mentioned above, that is, over the entire bandwidth of the current sensor.
To this end the sensor according to the invention is characterized by the fact that it comprises an auxiliary resistor which has the same value of resistance as the damping resistance of the secondary winding and is connected to the common branch point of the secondary winding and the measuring resistor, and a differential amplifier block arranged so as to measure the voltage appearing at the terminals et the secondary winding and to apply to said auxiliary resistor a voltage of the same size but opposite in direction to that appearing at the terminals of the secondary winding.